The prevalence of obesity among American children is at an all-time high, with 17% of 6-11 year-olds having high body mass index for their age. Child obesity is associated with poorer physical and mental health and social wellbeing. OBJECTIVES: The home is the most influential environment for health in middle childhood. The proposed research will examine understudied relationships between family structure, family activities, and children's nutrition and weight. AIMS: The study addresses the following specific hypotheses: 1. Family structure is associated with children's nutrition environment and consumption choices in elementary and middle school;2. Family structure is associated with children's weight trajectories and obesity risks;3. The influence of family structure on children's nutrition and weight trajectories is partly through family interactions;and 4. There are cultural and racial differences in the implications of home environment for children's nutrition and weight. American children experience a myriad of family structures in terms of parental co-residence (biological, step, single-parent families), co-residence with other non-parental adults, and co-residence with other children, and these diverse and shifting circumstances may affect nutrition and obesity risks. Previous studies have shown that these aspects of the home environment affect several child outcomes, including education, health behaviors, and mental and physical health. Family structure may also influence children's nutrition and obesity risks. We test the family interactions that are expected to affect children's weight, including meals, activity levels and rules, and consider whether some are more likely to provide health promoting interactions. In addition, we examine disparities in obesity risks between diverse origin children and explore whether these arise from systematic differences in home environments. DESIGN: This will be a secondary data analysis using the Early Childhood Longitudinal Study Kindergarten Class of 1998-99 (ECLSK), the largest national longitudinal study measuring child weight and height. METHODS: We use several methods with time-varying information on weight status, family roster, family activities and other covariates at 7 points over 9 years: first-difference regressions and hierarchical linear models will be used to explore relationships between changes in family structure and changes in weight and nutrition. Two-stage models and multilevel structural equation models will be used to examine how family structure operates through family interactions. SIGNIFICANCE: This study will provide empirical evidence about the effects of the home environment on children's obesity risks, which family activities hold the greatest promise for healthy growth, and strategies for incorporating the family in interventions. It will show whether demographic changes in the American family are affecting children's obesity risks and whether changes in family activities, such as shared meals, can indeed affect children's weight, as is the expectation of current recommendations. PUBLIC HEALTH RELEVANCE: Childhood obesity is associated with poorer physical and mental health and social wellbeing. Previous studies and interventions have indicated that the family environment is important for healthy weight in children, but in order to reduce increases in child obesity, a better understanding is needed of specifically how the family environment affects children's weight trajectories, which individuals within the home are important, and what home-based activities really affect weight and bring about weight change in children. This study will inform research and policy about the extent to which children's health behavior and weight are shaped by the family and about the expected value of involving the family or promoting family activities as part of interventions to prevent and reverse obesity among children. Public